It is known in the art that various products may be produced by reacting one or more reactants in the presence of an metal-organophosphorus ligand complex catalyst. However, stabilization of the catalyst and organophosphorus ligand remains a primary concern of the art. Obviously catalyst stability is a key issue in the employment of any catalyst. Loss of catalyst or catalytic activity due to undesirable reactions of the highly expensive metal catalysts can be detrimental to the production of the desired product. Likewise degradation of the organophosphorus ligand employed during the process can lead to poisoning organophosphorus compounds or inhibitors or acidic byproducts that can lower the catalytic activity of the metal catalyst. Moreover, production costs of the product obviously increase when productivity of the catalyst decreases.
For instance, a major cause of organophosphite ligand degradation and catalyst deactivation of metal-organophosphite ligand complex catalyzed hydroformylation processes is due to the hydrolytic instability of the organophosphite ligands. All organophosphites are susceptible to hydrolysis in one degree or another, the rate of hydrolysis of organophosphites in general being dependent on the stereochemical nature of the organophosphite. In general, the bulkier the steric environment around the phosphorus atom, the slower the hydrolysis rate. For example, tertiary triorganophosphites such as triphenylphosphite are more susceptible to hydrolysis than diorganophosphites, such as disclosed in U.S. Pat. No. 4,737,588, and organopolyphosphites such as disclosed in U.S. Pat. Nos. 4,748,261 and 4,769,498. Moreover, all such hydrolysis reactions invariably produce phosphorus acidic compounds which catalyze the hydrolysis reactions. For example, the hydrolysis of a tertiary organophosphite produces a phosphonic acid diester, which is hydrolyzable to a phosphonic acid monoester, which in turn is hydrolyzable to H.sub.3 PO.sub.3 acid. Moreover, hydrolysis of the ancillary products of side reactions, such as between a phosphonic acid diester and the aldehyde or between certain organophosphite ligands and an aldehyde, can lead to production of undesirable strong aldehyde acids, e.g., n-C.sub.3 H.sub.7 CH(OH)P(O)(OH).sub.2.
Indeed even highly desirable sterically-hindered organobisphosphites which are not very hydrolyzable can react with the aldehyde product to form poisoning organophosphites, e.g., organomonophosphites, which are not only catalytic inhibitors, but far more susceptible to hydrolysis and the formation of such aldehyde acid byproducts, e.g., hydroxy alkyl phosphonic acids, as shown, for example, in U.S. Pat. Nos. 5,288,918 and 5,364,950. Further, the hydrolysis of organophosphite ligands may be considered as being autocatalytic in view of the production of such phosphorus acidic compounds, e.g., H.sub.3 PO.sub.3, aldehyde acids such as hydroxy alkyl phosphonic acids, H.sub.3 PO.sub.4 and the like, and if left unchecked the catalyst system of the continuous liquid recycle hydroformylation process will become more and more acidic in time. Thus in time the eventual build-up of an unacceptable amount of such phosphorus acidic materials can cause the total destruction of the organophosphite present, thereby rendering the hydroformylation catalyst totally ineffective (deactivated) and the valuable rhodium metal susceptible to loss, e.g., due to precipitation and/or depositing on the walls of the reactor. Accordingly, a successful method for preventing and/or lessening such hydrolytic degradation of the organophosphorus ligand and deactivation of the catalyst would be highly desirable to the art.